The proposed research will analyze the cellular development of embryonic hearts in the horseshoe crab and lobster. The primary objective is to determine which properties of the embryonic myocardial cells undergo alteration in the course of normal development. The heartbeat in adult horseshoe crabs and lobsters is initiated by pacemaker neurons located in the cardiac ganglion. A primary question is whether the initial heartbeats in these organisms are neurogenic or myogenic. The influence of the cardiac ganglion on myocardial cell properties will be studied in hearts that have just begun to beat and those later in development. Electrophysiological and ultrastructural properties of the myocardial cells will be analyzed. It is anticipated that these studies will increase our understanding of the normal events in the cellular development of myocardial cells and provide insights into the ways in which normal events may go awry in the course of embryonic development. BIBLIOGRAPHIC REFERENCES: Sherman, R. G. and T. G. Burrage (1976) Sarcomere Formation in the Embryonic Lobster Heart. Am. Zool. 16: 178. Burrage, T. G. and R. G. Sherman (1977) Development of the lobster Heartbeat. Federation Proc. 36: 443.